Soil organic carbon in sandy soils: A review

Abstract

Sandy soils cover approximately 900 million ha worldwide particularly in arid and semi-arid regions. There are extensive areas of sandy soils under cultivation, but the soil fertility is often low and dependent on the levels of soil organic carbon (SOC). Here, we review SOC levels of sandy soils across the world using pedon databases, data from literature, and three detailed case studies. Pedons were selected from five major databases and the pedons had a minimum of 850 g sand kg− 1 in the top 30 cm. Sandy soils occur in all climates and several soil orders (mainly Alfisols, Entisols, Inceptisols, Spodosols, Ultisols). Soil organic carbon was highest in sandy soils of the temperate and cold zones (mean 19 g kg− 1), and lowest in soils of the arid zone (< 5 g kg− 1). The SOC concentrations was highest when mean annual precipitation was between 700 and 1300 mm, and SOC was lower when annual temperatures were higher. Sandy soils under forest had on average 23 g SOC kg− 1 whereas the soils under prairie had on average 7 g SOC kg− 1. Spodosols had the highest SOC concentrations, and SOC was lowest in Aridisols. The average A-horizon thickness of sandy soils was 17 cm, and it was thickest in Mollisols, and thinnest in Inceptisols. The thickness of the A-horizon is important since most of the SOC of sandy soils is found in the A-horizon. Over 32,000 papers have been published on sandy soils since 1914, and in the past 10 years, some 1000–1800 papers are annually published on sandy soils. We extracted papers since the early 1990s, and grouped them by continent, land cover, soil order, and focus of the research. Three soil-forming processes have been extensively studied in relation to SOC and sandy soils: podzolization, humification, and melanization. Several studies found that SOC increases when sandy soils were fertilized, but sandy soils become more water repellent with an increase in SOC and a decrease in pH. From published literature, it was found that SOC was high in sandy soils under forest (≤ 90 g kg− 1) and grassland (≤ 187 g kg− 1), and low in sandy soils under agriculture (≤ 38 g kg− 1) or under shrub (≤ 36 g kg− 1). Lastly, we present case studies from Southern Africa, Western Europe, and North America. Sandy soils cover about 273 million ha in Africa, and SOC rarely exceeds 10 g SOC kg− 1 in the top 30 cm. The plaggen soils of Western Europe have high SOC (up to 66 g kg− 1) due to the long-term additions of sod, litter, manure, and sea sand to increase the soil fertility. In the Wisconsin Central Sand Plains, SOC stocks in soils under agriculture were high (60 Mg ha− 1) compared to the soils under forest (15 Mg ha− 1) and grassland (25 Mg ha− 1). Available water capacity increased with an increase in SOC. From this review, we conclude that sandy soils are found throughout the world, and the sandy soils in the temperate and cold zones have the highest SOC levels. In sandy soils, soil organic carbon increased the cation exchange capacity and lowered the bulk density. Soil organic carbon levels can be significantly increased in sandy soils under agriculture when amendments are made in combination with irrigation.